I've lived for over 10 years among chassidim, and do not remember ever seeing any chassid use the dual-sided implements, while the single-sided ones are a daily sight. My wife's Satmar/Malachim grandfather even promptly got us a stroller when he saw us using a child carrier!* I asked the son of a chassidic rebbe, who agreed that chassidim do not use such items, but was unable to explain it.

One may think that it stems from an aversion to modern trends, but the Bluetooth example would contradict this.

Where does this behaviour come from?

*It was clear that he was unhappy with the carrier, even though we were traversing stairs daily, using public transportation, and carrying luggage – all activities for which a carrier is more convenient than a stroller.

2 Answers
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The Gemara in Pesachim (109b) mentions that pairs (even numbers) are dangerous, while non-pairs (odd numbers) are fine. This is illustrated in a story involving Rabbah (BM 85a). Perhaps Chassidim feel that wearing two straps falls under this category and thus avoid it as a measure of safety.

(I have no idea if this is actually where it came from. I've never heard of such a minhag before. I'm positing a theory.)

According to US Army regulations, Soldiers may [only] carry these bags by hand, or on one shoulder using a shoulder strap. It would seem from this that dual-sided carrying is considered less dignified, and that would be sufficient reason for chassidim to disapprove.

The Air Force has similar rules, including leaving right hand free to salute. While chassidim do not salute, they do touch mezuzas with their right hand.

It may also be more uncomfortable to wear things like backpacks the more layers you are wearing on your torso. Military and chassidic uniforms each introduce additional torso layers.

I personally wear very light clothing, so I am less bothered by such issues. However, in yeshiva I used to wear a normal suit jacket, and my backpack would wrinkle my jacket and untuck my dress shirt.

So too, an over-the-head headset does not go well with wearing a hat, which many chassidim do throughout the day.

The above is further indicated by the fact that chasidim do wear shirts with two sleeves and glasses with two temples instead of sleeveless shirts and monocles.